EP0036540B1

EP0036540B1 - Mosaic printing head

Info

Publication number: EP0036540B1
Application number: EP81101683A
Authority: EP
Inventors: Gaboardi Angelo
Original assignee: Honeywell Information Systems Italia SpA
Current assignee: Bull HN Information Systems Italia SpA
Priority date: 1980-03-18
Filing date: 1981-03-07
Publication date: 1984-07-18
Also published as: EP0036540A2; IT1130552B; IT8020717A0; DE3164809D1; US4367962A; BR8101570A; EP0036540A3

Description

The present invention relates to a mosaic printing head with improved adjustment of the armature gap.
A mosaic printing head generally comprises a needle guide assembly and a plurality of electromagnetic structures coupled to the guide assembly.
The electromagnetic structures are mounted on an electromagnetic support and each structure includes a magnetic circuit formed by two pole pieces connected by a yoke and by a movable armature which completes the magnetic circuit.
It further includes at least a coil wound around one of the two pole pieces.
Each of the electromagnetic structures acts as an activator for one of the printing needles.
To this purpose each movable armature is provided with at least an arm which protrudes externally to the electromagnetic structure and operates as an actuating arm for the associated needle.
Examples of mosaic or matrix printing heads are given in English Patent n. 1.477.661, in U.S. Patent n. 4.051.941 and in Italian Patent Application n. 27496 A/78 filed by the same applicant.
With a matrix printing head the printing of each type is obtained by printing a dot composition.
The dot composition forming a type on the paper can be carried out both in one time and in subsequent times with an horizontal scanning of the type form.
In the first case 35 needles forming a matrix of 5x7 possible printing dots are generally provided.
In the second case a vertical column of 7 needles at least is generally provided which performs a type horizontal scanning.
It is clear that intermediate solutions are possible: the one, for instance, to provide two vertical columns of 7 needles each, the one next to the other, which perform a type horizontal scanning, halving in such way the printing time of a type.
For sake of simplicity the following description will refer to a mosaic printing head comprising 7 printing needles arranged to form a transversal column as to the horizontal shifting direction of the paper, while considering the solutions described in the following also valid for mosaic printing heads having an higher number of printing needles.
The printing needles and the respective electromagnets are generally mounted on a carriage, movable along the printing line, in order to execute the subsequent or "serial" printing of several characters; such needles and electromagnets form a printing head movable on a carriage operated by a step motor or by a c.c. motor.
The mosaic printing heads must be very unexpensive and simple in construction.
At the same time they must allow for precise adjustment, in particular of the air gap width in the electromagnetic structure with the armature in the rest position and therefore of the stroke of the associated printing needles.
Such needle stroke adjustment is essential to achieve a very high operation frequency because the maximum attainable frequency is greatly influenced and limited by such stroke.
In the already mentioned Patent Application n. 27496 A/78 filed by the same applicant a mosaic printing head is described where adjustment of the electromagnetic gaps in rest position is performed in collective manner.
In such case, however, as opposed to the undeniable advantages of a quick and easy collective adjustment of the air gaps, it is not possible to neglect how the working tolerances of both needles and electromagnetic structures can lead to stroke values very different for the several printing needles; this involves a type formation as much more defective as the printing speed is higher.
In fact, if a very fast matrix printing head supported by a carriage operated by a c.c. motor is considered and if in such printing head each needle has a different stroke, the transversal printing line corresponding to the seven needle line will not be perfectly aligned because the impact of the different needles against the printing support will not be simultaneous owing to the different strokes of the printing needles.
Therefore, for very high printing speeds, it is essential that the rest air gap adjustments be individually allowed for each electromagnetic structure and its respective needle.
A solution of this kind is described in U.S. Patent 4,140,406 where individual adjustment is provided by means of adjusting screws, screwed in an armature retaining and adjusting element. However, owing to the vibrations generated during the print head operation, the adjusting screws tends to unscrew and to modify the gap adjustment, unless the use of glue or lacquer is assumed for fixing the screw to the armature retainer, which use, besides involving a very problematical calibration, has the effect or inhibiting any further calibrations.
The printing head object of the present invention overcomes such inconveniences; in fact, while keeping a great structure simplicity, it enables a simple and fast adjustment of each electromagnet air gap in rest position eliminating any effect on the adjusting element produced by the vibrations occurring during the printing.
According to the invention these advantages are achieved by using an adjustment disk which may be metallic, on one side of which nearer the armatures a rubber ring of suitable thickness and hardness if vulcanized.
The metallic disk is provided with threaded holes, each one of which houses an adjusting screw.
Such screws protrude into corresponding unthreaded holes present in the vulcanized rubber ring and smaller than the external diameter of the screws.
In this way the rubber ring exerts a force on the, adjusting screws.
The force direction concides with the threaded hole axis while its sense prevents the screw from advancing.
In such way the axial lash between the hole threading and the adjusting screw one is recovered and, further, the vibrations have no effects on the screws.
Through such adjusting screw, the stroke of the printing needle coupled to it can be adjusted in a very precise manner.
These and other characteristics will appear more clearly from the following description of a preferred invention embodiment and from the enclosed drawings where:

- Figure 1 is a side view of a printing head according to the invention;
- Figure 2 is a perspective, exploded view of the elements which form a needle guiding assembly of the printing head according to the invention;
- Figure 3 is a perspective, exploded view of the elements which form an electromagnet assembly of the printing head according to the invention;
- Figure 4a is a front view and Figure 4b is a section view of the resilient ring with internal bumps of the electromagnetic assembly;
- Figure 5a is a front view and Figure 5b is a section view of the adjusting disk of the electromagnetic assembly;
- Figure 6 is a medium, side section view of the printing head according to the invention;
- Figure 7 is a front view of the inner side of the armature retainer used in the electromagnetic assembly;

With reference to Fig. 1, it shows in side view a printing head according to the invention comprising a needle guiding assembly 2 and an electromagnetic assembly 3.
The needle guiding assembly 2 comprises a frame 4 having a bracket 6, fixed to the frame by a screw 5, which supports a guide for the printing ends of the needles.
The guide for the printing ends of the needles is generally formed by a pair of suitably-shaped ruby plates 7, partially shown in Fig. 1.
The needle guiding assembly 2 is fixed to the electromagnetic assembly 3 by means of two screws (in Fig. 1 the head 8A of one of such screws is visible) which lock two ears present on the sides of frame 4 against the electromagnetic assembly; one of such ears is visible in Fig. 1 and numbered with 9.
The electromagnetic assembly 3 comprises a supporting disk 10, to which a suitable number of electromagnets such as 11, 12, 13, 14 are fixed, an internal bushing (not visible) fixed by calking or shrinkage in a central opening of disk 10, an armature retainer 16 and a central cap 17. A socket 18, provided with two side brackets (one of these referenced by 19 in Fig. 1) is fixed by screws (like 20 in Fig. 1) to the electromagnetic supporting disk 10.
For purposes of easier manufacturing of the elements, such socket is separated from disk 10 (as it will be seen in the following, disk 10 is preferably obtained by blanking), but it could be integral part of the same disk if this last one is obtained by means of other technologies, for instance casting.
In turn, socket 18, and consequently the whole printing head, is fixed to a printing carriage 21 by means of screws like screws 22 in Fig. 1.
The printing carriage is provided with driving bushings 23, 24 inserted onto two parallel guiding bars (visible in cross section in Fig. 1 as 25 and 26), so that the carriage and therefore the printing head can slide in the direction of said bars, perpendicular to the plane of the drawing.
The carriage and guiding bars are part of the printer frame, which is not shown being unessential to the understanding of the invention.
In Fig. 1 it can be seen that the external surface of the ruby plates 7 is placed at a distance D from a printing platen 27.
A paper printing support member 28 and an ink ribbon 29 are interposed between the platen and ruby plates.
Distance D must be suitably adjusted to leave a certain allowance between platen 27 and printing head, but at the same time it must be kept to a minimum in order to keep to a minimum the stroke of the printing needles.
It is known that high operating speed of the needles can be obtained only if the stroke of the needles is minimum.
At the same time the end of the needles at rest must not protrude beyond the rubies because it would cause needle wear and considerable wear of the inked ribbon.
Figure 2 shows in perspective exploded view and with greater detail the needle guiding assembly 2.
The needle guiding assembly 2 comprises a frame or elongated body 4 having a generally C-shaped section which is tapered towards one end 30 where bracket 6 is mounted.
Such frame, which is opened in the upper side as shown in Fig. 2 (but in alternative it could be opened on the lower side), may be obtained by casting or preferably by plastic molding.
Frame 4 is provided, on sides 31 and 32, with internal vertical grooves 33, 34, 35 intended to receive needle guiding diaphragms 33A, 34A, 35A.
One side 32 of the frame (but alternatively the other one or the bottom even) has an elongated opening 36 intended to receive a locking screw 38 engaging with a threaded seat 37 of bracket 6.
Opening 36 is elongated in a direction perpendicular to the plane of impression so that bracket 6 may be fixed to frame 4 at a variable and adjustable distance from the platen.
Each of the diaphragms 33A, 34A, 35A is provided with a number of openings equal to the number of needles to be driven, with the openings being disposed over a suitable area of the diaphragms as illustrated. The closer the diaphragms are to needle guiding 7 the closer the openings approximate a vertical distribution.
Guiding member 7 consists of two rubies 39 and 40 having a vertical slot in which the needles are inserted and guided in vertical alignment.
In Fig. 2 one needle 41 only is shown with a head 42.
For each needle a compression spring 15 is inserted between the needle head 42 and diaphragm 35A.
The springs tend to keep the needles in a rest, retracted position and the heads apart from the diaphragm.
Frame 4 is provided on the sides with two brackets 9, 43 each having an opening for receiving two screws 44 and 8 used to fix the needle guiding assembly 4 to the electromagnetic assembly 3.
On the opposite end of where bracket 6 is mounted, frame 4 extends with an appendix 4A, preferably with an externally cylindrical shape. This appendix fits in a corresponding central opening of the electromagnet supporting disk and internally to a bushing which is part of the electromagnetic assembly 3.
Fig. 3 shows in perspective exploded view an electromagnet assembly 3.
The electromagnet supporting disk 10 consists of a ring-shaped member in the form of circular segment having a central round opening 1 OA. Supporting disk 10 is for instance obtained by blanking.
It contains a suitable number (7 in Fig. 3) of rectangular openings 45, 46,... 51 respectively, each of which receives an appendix provided in each magnetic core.
The openings are radially distributed around the disk centre and are uniformly spaced in a convenient circular sector of the supporting disk.
Two threaded openings 52, 53 enable screws 44 and 8 (Fig. 2) to engage in said openings and to fix the needle guiding assembly to the electromagnet assembly.
Two other threaded openings 54 and 55 receive screws 56 and 20 which, by previous insertion in openings 57 and 58, provided in brackets 59 and 19 of socket 18, enable to connect such socket to supporting disk 10. Socket 18 is provided with two openings 60A, 22A, elongated in the direction perpendicular to the plane of impression and intended to receive screws 60 and 22 which are screwed in suitable threaded seats of the printing carriage.
This socket 18 (and the whole print head) is fixed to the printing carriage and allows for adjustment of the printing head from the platen.
A cylindrical bushing 61, internally hollow, is fixed by calking or shrinking in the central opening 10A of the support disk 10.
An armature retainer 16 is placed on the bushing plane surface opposite to the bushing end which is fixed to disk 10.
Retainer 16 has substantially the same shape as disk 10, that is a ring-shaped element in form of circular segment with a central opening 16A.
As shown in Fig. 7, the peripheral portion of retainer 16 is provided with a groove 62 receiving a resilient string 63.
The string may be a rubber string with round section.
Along the groove 62, retainer 16 is provided with posts 64, 65, 66, 67, 68, 69 which interpose between adjacent armatures and impose a radial distribution of the armatures.
Similar posts 70, 71, 72, 73, 74, 75 are provided internally at the periphery of the central opening of the retainer, with the double function of imposing a radial distribution of the armatures and providing reatiner 16 with a contact face to bushing 61.
Fig. 3 shows, as an example of embodiment of a plurality of electromagnets, an electromagnet 93.
Electromagnet 93 includes a magnetic core 94, which is formed by a pack of generally U-shaped magnetic sheets provided with an appendix 94C to be inserted in one of the rectangular openings of disk 10, for instance opening 48.
A coil 95 is wound around a column of the magnetic core.
The magnetic circuit is closed by a movable armature 99.
The movable armature has an extension arm 100 operating as a lever arm for the actuation of a needle.
Such arm 100 is inserted between two internal posts (for instance 72, 73) of the retainer 16 (see also Fig. 7).
Opposite arm 100, the armature is provided with two grooves 101, 102 which receive the side portions of two adjacent external posts of retainer 16, for instance posts 66 and 67 of Fig. 3 and Fig. 7.
In this way, armature 99 and the other like armatures of the electromagnet group are precisely positioned in their respective seats in retainer 16.
Retainer 16 is fixed to bushing 61 by means of a ring-shaped cap 17.
Cap 17, generally of aluminium, has 3 openings 80, 81, 82 distributed on its peripheral portion, each one for receiving a screw 83, 84, 85 respectively, which is screwed into corresponding threaded seats of bushing 61.
Such screws are inserted in the three openings 86, 87, 88 in a resilient ring 89, which is interposed between cap 17 and retainer 16, as well as in the three openings 90, 91, 92 (Fig. 7) in the armature retainer, in correspondence with three internal posts.
The resilient ring 89, whose frontal view is shown in Fig. 4a and whose section view according to plane AA' is shown in Fig. 4b, is provided, internally, with 7 bumps 103, 104, ... 109.
These bumps extend in the central openings 16A of armature retainer 16 and define the rest position of the electromagnet armatures; such bumps further act as rebounding dampers of the armature stroke when they return in rest position.
The thickness of the bumps, as shown in Fig. 4b, is suitably greater than the thickness of the resilient ring 89 in order to enhance this damping effect.
Each bump is rear supported by an adjusting screw 115, 116,... 121 respectively.
These screws are housed in threaded seats of cap 17 as clearly shown in Fig. 6.
Cap 17, whose front view is shown in Fig. 5a and whose section view according to plane BB' is shown in Fig. 5b, has on the inner part of one face a circular recess where a rubber ring 122 of suitable hardness is vulcanized.
Cap 17 and rubber ring 122 are provided with openings numbered from 108 to 114, located in correspondence to bumps 103, 104,...109 of the resilient ring 89 and receiving screws 115 ... 121.
Such openings are threaded internally to cap 1 but not inside the rubber ring where their diameter is equal to the internal or minimum threading diameter.
This is obtained with no difficulties as the threading instrument does not bite the resilient material of ring 122.
As shown in Fig. 6, the adjusting screws extend beyond rubber ring 122 and adjust the position of bumps 103, 104... 109, and consequently the position of the armature and the width of the magnetic circuit air gap.
With this embodiment, a force exerted by rubber ring 122 is always acting on the adjusting screws. The direction of such force coincides with the threaded opening axis while its sense opposes to the advancement of the screw.
In such a way any movement between screw and nut thread is prevented owing to the interference between screw and opening in the vulcanized rubber; the sense of the force exerted by the vulcanized rubber ring grants a steady contact between screw and nut thread.
The whole assembly and the function of the several elements appears more clearly from Fig. 6 which is a median section view of the printing head taken from the same point of view of Fig. 1.
Figure 6 shows clearly a section of the needle guiding assembly with frame 4, needle guide 7 and diaphragms 33A, 34A, 35A.
Appendix 4A of the frame is inserted in bushing 61 which in turn has its end portion 61A inserted in the central opening of supporting disk 10.
Supporting disk has rectangular openings for mounting of the electromagnets.
In Fig. 6 an opening is shown, which receives appendix 94C of magnetic core 94.
A coil 95 is wound around one of the columns of the magnetic core and preferably, for avoiding encumbrance problems, on the external one, having in mind the radial mounting of the electromagnets.
The armature 99, associated with magnetic core 94, lies on the core at point 123 of the air gap and is kept in contact with the core by resilient string 63 inserted in groove 62 of retainer 16.
As shown in Fig. 7, the radial position of the armature is provided by posts 67, 66 inserted in two symmetrical grooves formed in the armature sides, and by the internal posts 72, 73.
Clearly, the other armatures, not shown in the drawings, are retained in an identical way.
Reconsidering Fig. 6 it is shown that armature 99 is kept away from the magnetic pole, in correspondence with the inner column, by the combined effect of spring 124 action on head 125 of needle 126 and therefrom on arm 100 of the armature as well as by the action of resilient ring 62 on the opposite end of armature 99.
The rest position of the armature is defined by bump 106 of resilient ring 89, and by the corresponding screw of cap 17.
It is clear that other armatures, not shown, are retained in rest position in an identical way.
As clearly shown in Fig. 6, retainer 16 is firmly pressed against the bushing 61 by cap 17 which is fixed to the bushing 61 by means of screws (in Fig. 6, screw 83 is shown).
Resilient ring 89, interposed between cap 17 and armature retainer 16, inserts its bumps in the central opening of armature retainer 16.
An adjusting screw on cap 17 corresponds to each bump.
The advancement or the regression of each adjusting screw varies the rest distance of the associated armature from the magnetic pole.
The rubber ring 122 vulcanized on cap 17 grants an high dynamical stability of the adjusting screws.
With the armature adjusting system described in the present invention the calibration of the needle stroke is both easy and fast.
In fact the calibration is made on inspection bench lining up the needle bits as to a vertical reference line, by means of optical devices.
This is performed by acting on the adjusting screws.
In alternative a calibration can be made on inspection bench as to the actuation time of several needles by detecting the impact instant by means of a piezoelectric transducer and therefore acting on the adjusting screws of the several needles in order to equalize their impact times.

Claims

1. A mosaic printing head which includes a plurality of electromagnetically actuated impact needles (41), each needle being actuated by a related elongated armature (99), the rest position of each of the armature being defined by a bump (103 ... 109) of a resilient ring (89), bearing a pump for each armature, the position of each bump being adjusted in position by a rear adjusting screw (115 ... 121) housed in a threaded seat of an adjusting disk (17), characterized in that said adjusting disk (17) is provided with a vulcanized rubber ring (122) on the side of said adjusting disk nearer the armatures and each of said threaded seats in said adjusting disk extends to an unthreaded opening (108... 114) of said vulcanized ring, the diameter of said openings being less than the external diameter of said threaded seat.

2. A mosaic printing head as per claim 1, characterized in that said bumps of said resilient ring, acting as damping elements for said lever arms, have a thickness greater than the thickness of said resilient ring.